Building structures having horizontal-floors and upright open-walls oftentimes include an upright slidable-door horizontally movable across the wall-opening of an upright-wall having opposed upright surfaces. It is customarily required that the temperature-gradient between the upright-wall opposed parallel surfaces be protectively minimized whenever the slidable-door is placed horizontally across the wall-opening, and in this regard, prior art workers have sought to attain a tight opening-obstructed contact between the slidable-door's upright rearward-plane and the upright-wall peripherally about the entrance/egress wall-opening thereof.